Long-term social bonds promote cooperation in the iterated Prisoner’s DilemmaIntroductionMaterial and MethodsSubjectsApparatus and experimental procedureTrainingTreatmentsAnalysesResultsDiscussionThe research presented here was carried out under animal care permit 06-042 provided by Université de Montréal and conforms to guidelines of the Canadian Council for Animal Care.We thank L.-A. Giraldeau, associate editor R. Bshary and two anonymous referees for their constructive comments on the manuscript. We are grateful to D. Drullion for her assistance and advice during the experiments. During this study, A.St-P. and K.L. were supported by research grants awarded to F.D. by the Natural Sciences and Engineering Research Council of Canada and Fond Québécois de Recherche sur la Nature et les Technologies.Referencesdoi: 10.1098/rspb.2009.1156, 4223-4228 first published online 9 September 2009276 2009 Proc. R. Soc. B Angèle St-Pierre, Karine Larose and Frédérique Dubois Prisoner's DilemmaLong-term social bonds promote cooperation in the iterated Referenceshttp://rspb.royalsocietypublishing.org/content/276/1676/4223.full.html#ref-list-1 This article cites 34 articles, 11 of which can be accessed freeSubject collections (1129 articles)behaviour  Articles on similar topics can be found in the following collectionsEmail alerting service hereright-hand corner of the article or click Receive free email alerts when new articles cite this article - sign up in the box at the top http://rspb.royalsocietypublishing.org/subscriptions go to: Proc. R. Soc. BTo subscribe to This journal is © 2009 The Royal Society on March 18, 2010rspb.royalsocietypublishing.orgDownloaded fromLong-term social bonds promotecooperation in the iteratedPrisoner’s DilemmaAnge`le St-Pierre, Karine Larose and Fre´de´rique Dubois*De´partement des Sciences Biologiques, Universite´de Montre´al, CP 6128, succursale Centre-ville,Montre´al, Quebec, Canada H3C 3J7Reciprocal altruism, one of the most probable explanations for cooperation among non-kin, has beenmodelled as a Prisoner’s Dilemma. According to this game, cooperation could evolve when individuals,who expect to play again, use conditional strategies like tit-for-tat or Pavlov. There is evidence thathumans use such strategies to achieve mutual cooperation, but most controlled experiments with non-human animals have failed to find cooperation. One reason for this could be that subjects fail to cooperatebecause they behave as if they were to play only once. To assess this hypothesis, we conducted an exper-iment with monogamous zebra finches (Taeniopygia guttata) that were tested in a two-choice apparatus,with either their social partner or an experimental opponent of the opposite sex. We found that zebrafinches maintained high levels of cooperation in an iterated Prisoner’s Dilemma game only when interact-ing with their social partner. Although other mechanisms may have contributed to the observed differencebetween the two treatments, our results support the hypothesis that animals do not systematically give into the short-term temptation of cheating when long-term benefits exist. Thus, our findings contradict thecommonly accepted idea that reciprocal altruism will be rare in non-human animals.Keywords: Iterated Prisoner’s Dilemma; cooperation; reciprocal altruism; social bond;zebra finch; tit-for-tat1. INTRODUCTIONOver the last four decades, extensive theoretical work hasbeen conducted on the Prisoner’s Dilemma (PD), a two-player model of the evolution of non-kin cooperation thatincludes two choices: cooperate or defect (Trivers 1971).In this game, it is always best to defect, no matter whatthe opponent does. Consequently, the expected outcomeis mutual defection if the opponents interact only once.However, because players get a greater payoff frommutual cooperation than from mutual defection,Axelrod & Hamilton (1981) suggested that cooperationcould emerge and be maintained when the game isrepeated such that the opponents, who expect to playtogether for an unknown number of interactions, adopta conditional strategy like tit-for-tat (TFT). TFT playerscooperate in the first round and then copy theiropponent’s previous move on all subsequent plays.Since the original formulation of this game, many theor-etical models of reciprocity have been developed toimprove its realism. Some of them, for instance, haveincluded the possibility that players make mistakes andreport that other conditional strategies like a generousTFT (Nowak & Sigmund 1992) or Pavlov (Nowak &Sigmund 1993) could then lead to stable mutualcooperation. Both strategies outperform TFT becausethey can correct occasional mistakes: generous TFTplayers cooperate with a certain probability after theopponent’s defection whereas players using Pavlov—referred to as a ‘win –stay, lose–shift’ strategy—keep thesame play when rewarded but change when punished.Although there is evidence that humans use such reac-tive strategies to achieve mutual cooperation in gameswith a PD-like payoff structure (Wedekind & Milinski1996; Milinski & Wedekind 1998), available data suggestthat reciprocal events will be rare in animals. Indeed, anumber of experimental and field studies with fishes (e.g.Millinski 1987; Ward et al. 2002; Bshary et al. 2008),birds (e.g. Godard 1993; Olendorf et al. 2004; Kramset al. 2008), vampire bats (Desmodus rotundus; Wilkinson1984) and primates (e.g. Packer 1977; Hauser et al.2003) have found apparent evidence for reciprocity. Inmost cases, however, the situation resembles a PD butthe fitness costs and benefits of cooperation are notprecisely measured and hence remain ambiguous. Inaddition, as reciprocal events frequently occur among kinin natural situations, alternative explanations cannot gener-ally be ruled out. To counter theses problems, severallaboratory experiments with controlled payoff games havebeen conducted, but most of them failed to find cooperation(Gardner et al. 1984; Reboreda & Kacelnik 1993;Clements & Stephens 1995; Green et al. 1995; Hall2003; Stevens & Stephens 2004; but see Stephens et al.2002, 2006). One possible explanation for the fragility ofcooperation in the iterated Prisoner’s Dilemma (IPD) isthat subjects fail to cooperate because they expect thattheir chances of encountering the same opponent laterare so low that they behave as if they were to play only asingle round of the game. Indeed, the success or failure ofcooperation in the IPD is